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The biomass balancing act

Specifying a heating system that includes both conventional condensing and biomass boilers involves a difficult balance in terms of controlling temperatures. Steve Cooper explains
Most system designs incorporating biomass boilers also feature gas condensing boilers for peak loads and standby back-up. Full optimisation of both these technologies involves something of a balancing act. However, the technologies have different optimum operating temperatures. Biomass boilers typically want to run at 80 deg/60 deg C. But that return temperature is far from ideal for gas condensing boilers.

Gas-fired condensing boilers deliver efficiency by extracting latent heat from the flue gases. This is done by dropping the temperature of the flue gases below the dewpoint which, for natural gas, is around 54 deg C. So, to operate in condensing mode, a return temperature of 54 deg C or below is required. More effective maximum system temperatures would therefore be 70 deg C supply and 50 deg C return or, to condense continuously, 50 deg C supply and 30 deg C return.

When both technologies are installed alongside one another, the biomass camp usually wins because of the technical problems of operating below 60 deg C. Thus, the system is designed, at best, for 80 deg/60 deg C or, at worst, for 80 deg/70 deg C even though there are usually variable or low temperature circuits which require far less.

So we might see a system design like Figure 1. This system will work, but will cost more to install and won't operate at optimum efficiency. A 400kW system, designed at 80 deg/70 deg C, would cost around 19 per cent more to install (because of larger pipe and pump sizes) and around 13 per cent more to run, than a truly integrated low and zero carbon system which operates each sub-system at its optimum operating temperatures.

There is a better alternative, however, as can be seen in Figure 2. Here the biomass boiler feeds a buffer vessel. On start-up, the hot supply water is diverted via the three-port valve into the biomass boiler return. Once the return water reaches 60 deg C, the three-port valve starts to close to the by-pass and allows water through to the buffer vessel and eventually heats it to 80 deg C. The pump on the CT circuit draws water from the buffer vessel through the three-port valve which is normally closed to the by-pass port.

The gas-fired condensing boilers serve the VT circuit(s) with LTHW at 65 deg/45 deg. A three-port mixing valve and pump on each VT circuit reduces the system temperatures further as required. Normally, the biomass and condensing boiler systems are separated by the threeport diverting valve on the common flow header. However, in the event of a failure of either, the three-port valve will open to allow flow from one to the other.

Optimal efficiency also relies on the control strategy for the condensing boilers, bearing in mind the part load environment. A typical control strategy would be to stage each boiler up to full load and then sequentially bring in each of the other boilers until the load is satisfied.

That works well for traditional boilers, because we're switching them in and out at their peak efficiency point. But, for condensing boilers, it's far from perfect. Condensing boilers are at their most efficient at part load not full load.


The reason is that, while the heat exchanger surface remains constant, the fuel/air mixture is variable. On low loads a greater surface area is available to extract heat energy from the reduced gas/air mix. So sequencing condensing boilers at peak load actually switches them at their least efficient point.

A better strategy is to run all multiple condensing boilers simultaneously at their lowest possible load to meet the building demand. This offers demand based control as opposed to the more widely used capacity based control. As well as being more energy efficient, demand based control reduces wear and tear on boilers from constant stopping and starting. It also avoids wasting energy during the pre-purge sequence necessary each time the boiler is restarted. The pre-purge sequence ensures that unburnt fuel and products of combustion are purged from the system. To achieve this, the fan blows cold air through the heat exchanger and up out through the flue, losing heat on the way. While energy wastage is small, the costs mount up throughout the boiler's life.

In recent projects involving biomass, there has also been a desire to incorporate solar technology. For example, the energy centres for two new academies in the Sunderland area - Academy 360 and Red House Academy - each incorporate a 350 to 500kW biomass boiler fuelled by wood pellets, with additional heat generated by solar panels.

With effective integration, there is no reason why all three technologies cannot be operated successfully, with the capabilities of each optimised. For example, Armstrong Integrated has developed preconfigured LZC solutions which combine biomass boilers, heat pumps, solar thermal and condensing boilers into systems that work at optimum temperatures and optimum efficiency for most of their working time. These incorporate two key components; thermal energy stores and demand-based, automation software.

A biodegradable propylene glycol solution is circulated through the solar array by the solar pump station once a predetermined temperature differential is established between TS1 and TS2. See Figure 3 - the solar array heats the water/glycol mixture which in turn heats the lower portion of the stainless steel solar cylinder. When there is insufficient solar energy available to achieve 60 deg C at TS3, the LZC controller automatically brings in the back-up boiler for instant top up. The LZC controller uses application-specific automation software to ensure each sub-system and each piece of equipment runs at the required load and temperature for optimum efficiency.

As the range of technologies available to system designers widens, integration will become increasingly important. Establish the optimum boiler control strategy and the remaining pieces of the puzzle begin to fall into place.

// The author is director - sustainable design at Armstrong Integrated //
1 February 2012

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